This invention relates to an antenna unit for receiving satellite signals transmitted from artificial satellites and, in particular, relates to an antenna unit mounted in a narrow-mounting-space cabinet such as a cabinet for an outside mirror of an automobile.
Various antenna units for receiving satellite signals transmitted from artificial satellites have been proposed. For example, such antenna units include a GPS antenna unit for receiving GPS signals transmitted from GPS satellites and a SDARS antenna unit for receiving SOARS signals transmitted from SDARS satellites.
For instance, in recent years, the so-called global positioning system has been spreading in which a receiver receives signal waves transmitted respectively from a plurality of artificial satellites orbiting the earth and the current position of the receiver itself is detected based on information included in the received signal waves. This system is generally called a GPS (Global Positioning System) in those countries such as Japan and USA. The GPS generally uses GPS satellites controlled by the US Department of Defense. As similar systems, there are “GALILEO” used in Europe and “GLONASS” used in Russia. Herein, a positioning system using artificial satellites, the artificial satellites used in the positioning system, signal waves transmitted from the artificial satellites, receivers for receiving the signal waves, and so on are referred to as a GPS, GPS satellites, GPS signals, GPS receivers, and so on, respectively, for convenience sake.
The GPS is capable of detecting a current position of a GPS receiver itself with high accuracy and substantially in real time. Accordingly, the GPS is mainly used such that a GPS receiver is mounted in a moving object such as an automobile, an airplane, or a portable telephone and the current position of the moving object is measured.
Presently, GPS receivers that are suitable when installed in automobiles, i.e. so-called car GPS receivers, are rapidly spreading. When installing the GPS receiver in the automobile, a GPS receiving antenna unit for receiving GPS signals is often disposed outside the automobile, for example, on a roof.
On the other hand, the SDARS (Satellite Digital Audio Radio Service) is a radio service according to a digital radio broadcasting using artificial satellites (which will called “SDARS satellites” hereinafter) in the United States of America. That is, in recent years, a digital radio receiver, which receives the satellite wave from the SDARS satellites or the terrestrial wave so as to listen to the digital radio broadcasting, has been developed and is put to practical use in the United States of America. Specifically, two broadcasting stations called XM and Sirius provide radio programs on 250 or more channels in total. The digital radio receiver is generally mounted on a mobile object such as an automobile and is adapted to receive a radio wave having a frequency of about 2.3 gigahertz (GHz) as a received wave to listen to the digital radio broadcasting. In other words, the digital radio receiver is a radio receiver capable of listening to mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance frequency) λ thereof is equal to about 128.3 mm. It is noted here that the terrestrial wave is a radio wave obtained by receiving the satellite wave at a ground station, slightly shifting the frequency of the satellite wave, and retransmitting the linear polarized wave. Thus, the terrestrial wave is the linear polarized wave exhibiting linear polarization while the satellite wave is a circular polarized wave exhibiting circular polarization.
An XM satellite radio antenna apparatus normally serves to receive circular polarized radio waves from two stationary satellites and, in an insensitive zone of the circular polarized waves, receives a radio wave by using a terrestrial linear polarization portion of the radio antenna apparatus. On the other hand, a Sirius satellite radio antenna apparatus normally serves to receive circular polarized radio waves from three orbiting satellites (synchronous type) and, in the insensitive zone, receives a radio wave by a terrestrial linear polarization portion of the radio antenna apparatus.
As described above, the radio wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting. Therefore, an antenna for receiving the radio wave is often located outside as known in the art. If the digital radio receiver is mounted in the mobile object such as the automobile, the antenna unit is often attached to a roof of the mobile object (car body).
Various such antenna units for receiving the satellite signals transmitted from the artificial satellites have been proposed. For example, Japanese Patent No. 431-4486, which will be called Patent Document 1, discloses an antenna unit which is capable of easily positioning a packing member to a top cover. In addition, Japanese Unexamined Patent Publication Tokkai No. 2006-245719, namely, JP-A 2006-245719 (which corresponds to U.S. Pat. No. 7,339,538), which will be called Patent Document 2, discloses an antenna unit that is excellent in assembly performance which assembling the antenna unit. Furthermore, Japanese Unexamined Patent Publication Tokkai No. 2006-237951, namely, JR-A 2006-237951, which will be called Patent Document 3, discloses an antenna unit which is capable of improving a waterproofing function. Such as antenna units mounted on the outside such as the roof of the mobile object will be later called “roof-mounted antenna units.”
On the other hand, Japanese Unexamined Patent Publication Tokkai No. 2008-78901, namely, JP-A 2008-78901, which will be called Patent Document 4, discloses an antenna unit which is housed in an outside mirror of an automobile. Such an antenna unit housed in the outside mirror of the automobile will later be called an “outside mirror housed antenna unit.” Inasmuch as the outside mirror has an open/close structure for opening and closing the outside mirror against a car body of the automobile, a lot of pars such as a motor drive, a motor, and so on are housed in the outside mirror. Therefore, a gap which is capable of using in the cabinet of the outside mirror is very narrow. That is, the cabinet of the outside mirror is a cabinet having a narrow mounting space that will later be called a “narrow-mounting-space cabinet.”
In such an outside mirror housed antenna unit, a waterproofing mechanism is required because the outside mirror is weather-damaged. Hence, it is considered that the roof-mounted antenna unit disclosed in the above-mentioned Patent Documents 1-3 is used in as the outside mirror housed antenna unit. However, the roof-mounted antenna unit requires a permanent magnet for fixing the roof-mounted antenna unit in question to the roof of the automobile by magnetic attraction while the outside mirror housed antenna unit does not require the permanent magnet. On the other hand, the outside mirror housed antenna unit requires a bracket for mounting the outside mirror housed antenna unit in question in the cabinet of the outside mirror.
In the manner which will later be described in conjunction with FIG. 1, in an antenna unit related to this invention, a gasket is sandwiched between a top cover and a bottom plate and is pressure inserted therein by securing the gasket using three screws. Thereafter, the bracket is mounted to the bottom plate using a plurality of screws. Therefore, the related antenna unit is disadvantageous in that the number of parts is increased and the number for mounting is increased.